1. Field of the Invention
The present invention relates to an image heating apparatus and a heater for heating suitably used as a heat-fixing apparatus to be mounted on a copying machine, a printer or the like using a recording technology such as an electrophotographic type or an electrostatic recording type recording technology, and more particularly to an apparatus using a metal substrate as a substrate of a heater.
2. Related Art
Conventionally, a heat roller type heating device has been extensively used in a device for heat-fixing processing as a permanently fixed image on a recording material surface an unfixed toner image corresponding to target image information formed in a direct manner or an indirect (transfer) manner on a surface of a recording material (an electro-facsimile sheet, an electrostatic recording sheet, a transfer material sheet, a printing sheet or the like) by using toner made of thermally meltable resin or the like by suitable image forming process means such as an electrophotographic recording technology, an electrostatic recording technology, a magnetic recording technology or the like, i.e., a heat-fixing apparatus in an image forming apparatus such as a copying machine, a printer, a facsimile or the like using, for example, an electrophotographic system.
The above-described heat roller type is basically composed of a roller made of metal and provided therein with a heater and a pressure roller having elasticity, which is brought into a press-contact with the roller. The recording material is caused to pass through a fixing nip portion defined by a pair of these rollers, whereby an unfixed toner image borne on the recording material is heated and pressurized to be fixed.
Also, the present applicant previously proposes a film heating type heating device in Japanese Patent Application Laid-open No. 63-313182 or the like.
According to this film heating system, a heater (heating body) and a heated member are respectively brought into contact on one side and the other side of a heat resistant film so that a thermal energy of the heater is given to the heated member via the heat resistant film. It is possible to use a film or a heater having a low heat capacity. Accordingly it is possible to shorten wait time (quick start, on-demand fixing) in comparison with the conventional heat roller type heating device.
Also, the quick start is possible to thereby dispense with preheat upon the non-printing operation and it is possible to save electric power in a total sense.
FIG. 6 is a schematic structural model view (cross-sectional model view) of a typical example of a heat-fixing apparatus using a film heating system. This apparatus is composed of a ceramic heater 7 as a heating body, a stay 13 that is a support member for supporting and insulating the heater 7, a cylindrical film 12 made of heat resistant resin, which surrounds loosely the stay 13 for supporting the heater 7, a pressure roller 9 being in press-contact with the heater 7 with the film 12 interposed therebetween for defining a nip portion N, and the like.
The pressure roller 9 is rotated in a counterclockwise direction indicated by the arrow by means of drive means M. With the rotation of the pressure roller 9, a rotary torque is applied to the film 12 by a frictional force between the pressure roller 9 and the film 12 in the nip portion N so that the film 12 is kept under the condition that it is accordingly caused to rotate in the clockwise direction indicated by the arrows about the stay 13 with its inner surface in sliding contact with the heater 7 surface. The stay 13 serves also as a guide member for the rotating film 12.
Under the condition that the pressure roller 9 is drivingly rotated, the film 12 is driven in accordance with this rotation and an electric power is fed to the heater 7 so that it is heated to a predetermined fixing temperature under the control, a recording material P to be fixed with an image as the heated member to be conveyed from a recording portion of an image forming apparatus (not shown) is introduced between the film 12 of the nip portion N and the pressure roller 9 to be conveyed while being clamped together with the film 12 through the nip portion N, whereby the heat of the heater 7 is given to the recording material P via the film 12 to soften an unfixed image (toner image) t to the surface of the recording material P to perform the heat fixing of it. The recording material P that has passed through the nip portion N is conveyed and separated in accordance with its curvature in order from the surface of the film 12. In order not to adhere the unfixed toner on the surface of the film 12, a heat resistant releasing layer made of fluorine resin or the like that is superior in releasing property is provided thereon.
FIGS. 7A to 7C are views showing a structural example of the ceramic heater 7 as a heating body. FIG. 7A is a schematic partially fragmental plan view of a front surface side of the heater. FIG. 7B is a schematic plan view of a rear surface side of the heater. FIG. 7C is an enlarged cross-sectional, schematic view of the heater.
The heater 7 is formed by laminating and baking in order by a screen printing technology a resistor pattern 2 heated by feeding electric power, a folded electrode 6, a power feeding electrode 5, a conductive pattern 5a that is an extended portion of the power feeding electrode 5 and a surface protective glass layer 3 on the front surface side of a ceramic substrate 1 such as alumina, aluminum nitride, silicon carbide or the like. A temperature detecting element (thermistor or the like) 4 is provided on the rear surface side of the ceramic substrate 1.
A power supply (AC input) is performed to the resistor pattern 2 through the power feeding electrode 5 and the conductive pattern 5a from a power feeding circuit (not shown) to thereby rapidly elevate a temperature of the heater 7 as a whole.
For the temperature control of the heater 7, the temperature detecting element 4 is brought into contact with a rear surface of the heater 7 so that the temperature is outputted as a voltage and furthermore, the output is calculated by a control circuit (not shown) such as a CPU to thereby adjust the AC input to the heater 7.
In this kind of conventional heating device, a ceramic heater using an alumina or tike as a heating body has been used. However, the device has suffered from problems in that the ceramic is fragile, a cost is high, the ceramic is not suitable for bending machining or the like.
Therefore, the present applicant proposes a heating device using a metal plate as substrate for heating body in Japanese Patent Application Laid-open Nos. 9-244442 and 10-275671 in advance. In this heating device, as a heating body, an insulating layer is formed on a metallic substrate to form the same substrate having the insulating property as the conventional ceramic substrate and a resistor pattern, a conductive pattern and an insulating sliding layer as an uppermost layer are formed thereon.
Thus, the substrate is made of metal to thereby enhance the mechanical strength of the heater.
On the other hand, in order to enhance the fixing property of the toner, it is proposed to provide an elastic layer on a film. In particular, in the case where the images of the overlapped toner layers as in the color image are to be fixed, it is possible to obtain the effect for surrounding the toner and the fixing property can be further enhanced by providing the elastic layer.
However, if the elastic layer is used in the film, the rigidity of the film is increased and the driving torque for the film is increased
Also, in FIG. 5 of the above-described Japanese Patent Application Laid-open No. 9-244442, the structure in which a nip surface side of the metallic substrate is formed into an arcuate shape is described.
However, since the opposite surface to the nip surface is flat, the thickness of the substrate is increased so that the responsibility of the temperature detecting element provided in the opposite surface side to the nip is degraded. For this reason, although in this fixing apparatus, it is easy to perform the fine adjustment of the temperature of the heater and it is possible to suppress the temperature ripple inherently, since the responsibility of the temperature detecting element is degraded, the temperature ripple is remarkable.